I will be traveling from Rome to Florence on 3/21, which is a date where I now know there is a planned rail strike. I looked at the government site, but I could not tell how the strike would affect my trip. It is the Trenitalia high speed train. How worried should I be about this?
High speed trains are usually not affected by strikes. You should be there a little earlier. Depending on the operational situation, trains sometimes leave earlier.
I would not. Strikes are frequently "discussed" but seldom enacted. And if they are, not all trains are effected. There are always few trains running. Be flexible and you will be OK.